The present invention relates to a drill string sub assembly comprising a mechanical filter.
Logging while drilling (LWD) apparatus is disclosed in U.S. Pat. No. 5,387,767 which enables acoustic signals to be transmitted to rock about a borehole, while minimising the energy transmitted into the drill collar. However, when drilling a borehole it may be desirable to know the stratum which is about to be drilled through, that stratum immediately ahead of the drill bit. This allows appropriate drilling parameters to be employed such as drill speed or the weight on the bit. This is particularly important when drilling for oil, for it enables over pressure regions in the stratum ahead of the drill bit to be identified and the fluid pressure within the borehole adjusted to a minimum safe level accordingly.
UK patent application, publication number 2288020 discloses a drill bit arrangement embodying an acoustic transducer which enables acoustic signals to be transmitted and received through the drill bit itself. This enables the region ahead of the drill bit to be surveyed without the very expensive requirement of withdrawing the drill string and lowering a wire-line tool down the borehole including an acoustic sensor to survey the bottom of the borehole. The apparatus described in UK patent application publication number 2288020 discloses that it is necessary to isolate the drill bit and acoustic transducer from the drill string in order to prevent excessive losses in transmitted and received signals from and to the transducer, caused by transmission along the drill string, and also to avoid problems caused by signals being returned along the drill string or reflected by the drill string.
This earlier application discloses two embodiments for isolating the drill bit and acoustic sensor from the drill string, and further development work on these resulted in the design illustrated in FIG. 1 of the attached drawings, the essential elements of which are briefly summarised below.
Referring to FIG. 1, a drill bit 10 is connected to a first hollow member 11 which slidably extends within second hollow member 12. Two sets of rubber coated splines 13 on the two respective hollow members engage and act to centralize the two members in the absence of any torque applied to the drill string, while two sets of steel splines 14 associated with each respective hollow member engage when high torque loads are applied during drilling to transmit torque from the drill string to the drill bit. A series of steel balls 15 engage with recesses 16 in the first hollow member and act to limit the relative separation of the two hollow members.
During a drilling operation the downward force exerted by the drill string causes the faces 17 of the two hollow members to come into contact such that the weight on the drill string is applied directly to the drill bit. When it is desired to survey the area of rock ahead of the drill string the weight on the drill string is reduced such that the first and second hollow members extend and adopt the position illustrated in FIG. 2. Compliances 18 acting between the first and second hollow members act to maintain a desired pressure on the drill bit to ensure good acoustic transmission between the drill bit and the rock surface it is in contact with, but in this position there is no direct metallic contact between the drill string and the drill bit. Piezo-electric elements 19 transmit a signal into the drill bit, the piezo-electric elements acting against reaction mass 20. After a short transmission signal a return signal is received via the drill bit and piezo-electric elements 21.
The problem with the arrangement illustrated in FIG. 1 is that it has to work in a very hostile environment in a borehole where various rubber components, seals and moving joints are subjected to very abrasive mud flow, high temperatures and pressures, and also a chemically hostile environment. This is particularly a problem because failure of the apparatus may result in a considerable down time while the drill string is removed and then replaced.